This invention relates to an electrical control system for an arc welding system and more particularly to a feedback control circuit for a pulsed direct current (DC) arc welding system. Specifically, this invention relates to a feedback control circuit for controlling power flow to work pieces at an arc gap by modulating the pulse width of current pulses supplied from a pulsed DC arc welding power supply to the arc gap. The current pulses are preferably modulated in response to resistance sensed at the arc gap.
There are many situations in which it is desirable to arc weld together two pieces of metal. For example, a heat exchanger for an air conditioning system may be made from sections of thin wall aluminum tubing which are joined to provide a continuous circuit for the flow of a refrigerant. The sections must be joined so that there are no leaks. One method for accomplishing this is by arc welding.
One problem encountered in arc welding is the presence of foreign materials on the surfaces of the work pieces which are being welded together. These foreign materials can degrade the quality of the weld if they are not removed. Metals such as aluminum, magnesium, and beryllium copper, pose an especially difficult surface contaminant problem since oxides instantaneously form on the surfaces of these metals when they are exposed to air. Oxides may be removed by using a nonmetal chlorine or fluorine base flux during the welding process but this flux is corrosive and is not compatible with the environment. Therefore, it is desirable to arc weld, especially to arc weld metals such as aluminum, magnesium, and beryllium copper without using a flux.
Fluxless welding is possible by using certain alternating current (AC) arc welding techniques. U.S. Pat Nos. 3,894,210 to Smith, et at. and 3,818,177 to Needham, et al. disclose such AC arc welding techniques. These techniques are especially useful for welding certain materials, such as aluminum, magnesium, and beryllium copper, since a weld can be made even if oxides are present on the surfaces of the work pieces. However, there are many situations when it is desirable to use direct current (DC) arc welding. For example, it is difficult to weld thin wall sections of aluminum tubing used in making heat exchangers for air conditioning systems by using an AC arc welding technique. This is because AC arc welding requires a significant power flow to the work pieces to make a weld and dissipate oxides without using a flux. This power flow heats the work pieces to an undesirable temperature because the thin wall tubing does not provide a sufficient heat sink for conducting away heat energy. Thus, significant sagging in the weld area can occur and there is a possibility that the work pieces will be burned through. This distortion of the weld area can be reduced if DC arc welding is used. Also, electrode life can be increased if DC arc welding is used rather than AC arc welding. Furthermore, power flow to the work pieces may be more precisely controlled when using DC arc welding. These are just some of the advantages inherent in DC arc welding when welding certain materials such as the thin wall sections of aluminum tubing used in making heat exchangers for air conditioning systems. Therefore, it is preferable to weld these materials by using DC arc welding rather than by using other techniques such as fluxless AC arc welding.
One disadvantage of conventional DC arc welding is that this type of arc welding is not generally capable of fluxless welding of certain materials, such as aluminum, magnesium, and beryllium copper, which form difficult to reduce oxides on their surfaces. However, there is a novel method of pulsed DC arc welding for welding these materials without using a flux. This novel method is desclosed in copending U.S. patent application Ser. No. 252,567, filed Apr. 9, 1981, in the name of Moyer et al., entitled "Pulsed DC Arc Welding". This copending application is assigned to the same assignee as the present application.
According to this novel method, special pulses of positive direct current are applied at an arc gap to arc weld work pieces at the arc gap. The special pulses have a form which is similar to conventional DC pulses except that the ratio of the magnitude of the peak current to the magnitude of the maintenance current at the leading edge of each current pulse is selected to have a special feature. Essentially, this ratio is maximized and the increase from the maintenance current level to the peak current value is adjusted to occur in a time interval whereby a thermal shock effect is created. A related kind of thermal shock effect is well known in the field of vacuum brazing as part of a multi-step heat treatment process in which materials are joined together by brazing. Basically, this thermal shock effect results from rapidly heating work pieces having surface oxides with a coefficient of thermal expansion which is substantially less than the coefficient of thermal expansion of the underlying pure material. The rapid heating causes an uneven rate of expansion which fractures and splits apart the oxides on the surfaces of the work pieces.
The split apart oxides are pushed away from the weld area due to the melting and joining of the underlying pure materials during the novel arc welding process disclosed above. Other physical phenomena also may be responsible for the exemplary welds formed when using this novel arc welding method but the thermal shock effect is believed to be the primary mechanism by which the oxides are dissipated. Regardless of the exact physical phenomena underlying the oxide dissipation, the feature of maximizing the ratio of peak current to maintenance current at the leading edge of each current pulse is an essential element of this novel method of DC arc welding. This feature is best explained when it is assumed that the thermal shock effect is the primary mechanism by which the oxides are dissipated.
The optimal values for the maintenance current, peak current and time duration in which the increase from the maintenance current level to peak current value occurs, when arc welding according to the novel arc welding method described above, are selected through a trial and error process. These optimal values depend on the kind of material being welded, the thickness of the work pieces being welded, and other such factors.
Also, power flow from the welding electrode to the work pieces is an important factor in determining weld quality. Good quality welds cannot always be made because of changes in this power flow as a function of time. It is especially difficult to continually make good quality welds on certain materials, such as thin wall aluminum tubing, when mass producing products, such as heat exchangers for air conditioning systems, because of this variation in power flow. This problem is present even if the novel method of fluxless pulsed DC arc welding described above is used in the manufacturing process.
These changes in power flow usually are caused by variations in the resistance between the welding electrode and the work pieces due to inhomogeneities in the ionized gas, variations in work piece dimensions resulting in a changing arc gap separation, naturally occurring fluctuations in power supply output voltage and other such phenomena. This variation in resistance between the welding electrode and the work pieces directly affects the amount of power which reaches the work pieces from the welding electrode. It is desirable to maintain this power flow at a constant optimal value since it is this power flow which primarily determines weld quality.
Conventional arc welding systems of the pulsed DC type do not specifically address the problem of controlling power flow to the work pieces. Typically, these systems regulate current flow by adjusting the voltage applied across the arc gap in response to variations in arc gap resistance to maintain the current flow at constant preset levels. Therefore, the normal operation of a current regulated pulsed DC system results in variations in the power flow to the work pieces.
A method of controlling this power flow, when using a pulsed DC power supply, is by changing the pulse width of the current pulses supplied to the arc gap. If a periodic series of current pulses is being used this amounts to changing the duty cycle of the current pulses. Thus, this method can be called pulse width modulation or duty cycle modulation. This method of controlling power flow is especially useful when the form of the DC pulses must be maintained as required when arc welding according to the novel pulsed DC arc welding method described above. Therefore, it is desirable to provide a control system for an arc welding system pulsed DC power supply which is capable of precisely adjusting power flow to work pieces by modulating the pulse width of current pulses supplied by the power supply to the work pieces. Preferably, this pulse width modulation is done without otherwise altering the general form of the current pulses. Furthermore, it is desirable to provide a control system for an arc welding system pulsed DC power supply which is capable of adjusting power flow by pulse width modulation to compensate for variations in resistance between the welding electrode and the work pieces.